This invention relates generally to nuclear reactors, and more particularly, to passive containment cooling systems in nuclear reactors.
One known boiling water reactor includes a reactor pressure vessel (RPV) positioned in a containment vessel and a passive containment cooling system (PCCS). The containment vessel includes a drywell and an enclosed wetwell disposed in the containment vessel. The PCCS includes a passive containment cooling condenser (PCC) submerged in a cooling pool located above the containment vessel.
In the event of a pipe break, steam generated by core decay heat is released from the RPV into the drywell. The steam has a pressure greater than the pressure within the wetwell and will, therefore, flow into the PCC inlet line carrying noncondensable gasses originally contained in the drywell. The steam is condensed in the condenser tube section, and the noncondensable gases are exhausted from the lower drum of the condenser via a gas vent line that discharges below the surface of the suppression pool in the wetwell. After rising through the suppression pool, the concondensable gases enter the wetwell air space above the suppression pool.
The condensate collected in the lower drum of the condenser drains to a drain tank or condensate storage tank via a drain line. A U-pipe loop seal or water trap restricts backflow of steam and noncondensable gasses in the drywell from flowing backwardly through the drain line back into the lower drum to bypass the condenser and enter the wetwell through the vent line. The drain tank has a separate injection line connected to the RPV to drain the condensate to the RPV. Inside the RPV, the condensate turns into steam by decay heat and the steam flows back to the drywell. This produces a continuous process by which the reactor core is cooled by water over a period of time following a pipe break.
However, the operation of continually returning noncondensible gasses to the wetwell results in a relatively high pressure in the containment. Also, incremental heating of the top layer of the suppression pool water each time noncondendsable gases are vented from the PCC into the wetwell can cause the pressure in the containment to slowly rise.
U.S. Pat. No. 5,282,230 to Billig et al. describes a bypass line connected to the drain line at a location above the U-pipe loop seal. The bypass line includes a normally closed bypass valve and discharges directly into the drywell. The bypass line channels the condensate and noncondensable gases from the lower drum through the top portion of the drain line and the bypass line for return to the drywell. Once the bypass valve is opened, the PCC operation relies on the natural circulation of steam being drawn into the condenser and the condensate falling by gravity back into the drywell. However, the long term removal rate of the PCC is controlled by the natural circulation, which can limit the rate of the post-accident recovery process. Also hydrogen gas generated by metal-water reaction in the reactor core could stay in the top portion if the intake pipe and PCC condenser and impede the natural circulation.
U.S. Pat. No. 6,097,778 to Cheung describes a passive gravity driven suction pump that converts the potential energy of the condensate to draw the condensable and noncondensable gases from the region downstream of the condenser tubes and discharge to the region outside the condenser. The advantage is that the device is passive, containing no moving parts, and does not use external power. However, the driving force, i.e., the potential energy, of the suction pump depends on the condensate drain rate. The condensate drain rate depends on the reactor decay power, which decreases over time after the reactor shut down, thereby, resulting in a reduced potential energy driving force of the suction pump.
It would be desirable to provide a containment cooling system for a nuclear reactor that has an enhanced flow through the condenser as compared to known passive containment cooling systems. Also it would be desirable to provide a containment cooling system for a nuclear reactor that effectively redistributes the noncondensible gases between the drywell and the wetwell.
In an exemplary embodiment, a nuclear reactor containment cooling system includes a containment vessel having a drywell and a wetwell, a cooling condenser submerged in a cooling pool of water located outside the containment vessel, a vent line extending from the condenser to a suppression pool disposed in the wetwell, and at least one drain line extending from the condenser to a condensate drain tank located in the drywell. The condensate drain tank includes a pool of water, and an end of the drain line is vertically submerged below the surface of the pool of water in the drain tank. To enhance flow through the condenser, a blower is located in the drain line.
In another embodiment, to enhance flow through the condenser, a drain line can include a jet pump apparatus. The jet pump apparatus includes a suction line, a pump in flow communication with the suction line, a discharge line extending from the pump to a jet pump nozzle located inside the drain line, and a venturi section located in the drain line The jet pump nozzle is positioned upstream from the venturi section.
In another embodiment, to enhance flow, the containment cooling system includes a drywell gas recirculation subsystem coupled to the vent line. The gas recirculation subsystem includes a suction pipe coupled to, and in flow communication with the vent line, at least one valve located in the suction pipe, at least one blower coupled to the suction line, and a discharge pipe in flow communication with the drywell.
The above described nuclear reactor containment cooling system has an enhanced flow through the condenser as compared to known passive containment cooling systems. Also, the above described containment cooling system effectively redistributes the noncondensible gases between the drywell and the wetwell.